Cooking, particularly frying and baking, releases air including fumes such as fat vapors, odor particles and/or hydrocarbons. Some fumes produced during cooking are desirable, since they are the source of appetizing aromas. However, other fumes are less desirable, particularly greasy vapors and smoke-like gaseous and particulate products, that accumulate on the inner surfaces of the oven, forming encrusted residues, and therefore require the oven to be frequently cleaned and maintained. If the fumes are simply vented outside the oven, as most known ovens suggest, the room rapidly becomes filled with fumes, which leave a film on the room's walls and ceiling and may also leave an odor that lasts for several days.
A known operation for cleaning ovens is treatment by pyrolysis that involves destroying the grease deposits or other residues that appear on the walls of an oven during cooking by a heat treatment. However, the burning of greases or stains again generates fumes, which may contain more or less harmful gases such as carbon monoxide.
It is thus desirable that harmful as well as unpleasant fumes be treated before they escape from the oven so that their presence does not jeopardize the atmosphere of the room.
Such fume and odor elimination in current cooking devices is typically handled by bleeding a small amount of the oven cavity air into a single pass, low temperature catalytic filter element, located between the cooking cavity and the appliance exhaust vent. In the catalytic filter element, airborne cooking fumes are oxidized into carbon dioxide and water, thereby preventing fumes and odors from escaping into the environment.
Such a catalyst filter element requires to be heated to a temperature of at least 180° C. before it can operate properly.
This is usually attempted by locating the catalyst filter assembly in the vent duct arrangement sufficiently close to the outlet from the oven that hot air from the oven heats the catalyst filter assembly.
Other technical solutions are known that provide a high temperature heating of the filter elements by individual heating means, and require the passage of electric current through heat-generating electrodes or heater coils for attaining a similar result.
One known design uses a catalyst supported on an electrically conductive wire, wherein the conductive wire is used as a resistive heater. But even when the wire is provided in a design comprising a plurality of wires arranged in a net like or screen like structure, a problem associated with this design is to keep the column of contaminated air in physical contact with the catalyst wire net for a period long enough for it to do its work as this design provides a low specific surface area.
Accordingly, catalyst coated wire screen design are generally inferior to a design that uses catalyst coated gas-permeable ceramics. Standard catalytic converters for self-cleaning ovens normally include a catalyst coated ceramic honeycomb-shaped disk. The honeycomb-shaped disk comprises a large number of cellular passages whose walls are coated with an appropriate catalytic agent for aiding in the combustion of the cooking vapors. For heating up to a selected temperature, the ceramic honeycomb disk is threaded with a high resistance electrical wire to provide an auxiliary heating means.
Also known from DE 4139 904 is an oven, provided with a concentric auxiliary heating element, which is surrounded by a catalytic converter assembly, wherein the catalyst is deposited as a coating on a foam ceramic of e.g. aluminum oxide or silicon carbide.
However, these designs, using a ceramic structure, always need an auxiliary electric heater, and thus the total electrical consumption of the oven is greater than for an oven with only a forced hot gas circulation heating system but without such catalytic filter elements. In addition, auxiliary heating up of the ceramic catalyst assembly can be insufficiently rapid, leading to release of undesirable smoke or fumes from the oven.
Moreover, the temperature distribution of a ceramic filter structure heated by an auxiliary heater may be non-uniform, resulting in narrowing of a region in which the burning of fumes occurs and thus difficulty in effective combustion of the cooking fumes. As a result, unburned residues may accumulate near an end surface of the filter element, flow paths openings on a filter inlet side may be clogged, resulting in a decrease in function.